Life support pigment in a nutrient limited ocean

Photorhodopsin is a light sensitive pigment, similar to that found in the human eye. Until recently, photorhodosin was only thought to be found in archaea, but using molecular analysis Beja et al (2001) indicates that a different and diverse form of photorhodopsin is widespread among bacteria. It has been shown that the pigment is indeed light sensitive and acts as a proton pump, but does not produce enough energy to fix carbon. The function of this pigment was therefore less well-understood, and subject to debate among scientists.

In 2010, Gomez-Consarnau and colleagues discovered that photorhodopsin functioned as a solar powered booster pack, promoting survival when nutrient limitation threatened. The bacterium they isolated was a Vibrio sp. (87% similarity with V. harveyi), the first Vibrio. sp  to be identified with the photorhodopsin gene (PR). Interestingly, phylogenetic analysis clustered the PR gene with Alphaproteobacteria, showing that the PR gene was acquired through horizontal gene transfer.

To draw their conclusion, they used experimental methodologies backed up using a genetic-mutation. Firstly, a starvation experiments incubated bacteria in different light treatments: continuous light; light/dark cycles - high light (representing surface waters), low light (representing deeper water); no light, without nutrients. Bacteria starved in continuous or high light had grown 40-60% more in size and was twice as abundant than bacteria without light, suggesting that photorhodopsin was improving survival during starvation. Furthermore, deletion of the PR gene (PR mutant) supported the role of photorhodopsin as a light-harvesting pigment, as PR mutants did not show any variation in survival fitness between the light treatments. Additionally photorhodopsin helped the bacteria recover from starvation, as bacteria supplied with nutrients after starvation had a 3-5 fold higher density in the light compared to those in the dark, and PR mutants remained unresponsive to different light treatments. This shows that energy provided by the photorhodopsin helped bacteria respond to improved conditions, further promoting their survival.

The abundance and diversity of the PR gene is not surprising given its function. Nor that this gene is passed horizontally, as it will provide a huge advantage in nutrient poor open water. However, I question the necessity of this gene in the Vibrio and think it may just reflect Vibrio’s opportunistic nature. Copiotrophs cope with nutrient extremes by reducing and expanding their size, further, they express rapid chemotaxis in order to source nutrients, which gives them enhanced advantages in locating food anyway. However, Vibrio are hugely successful and dominate due to this opportunistic nature, plus if this gene was ‘available’ I can’t say I wouldn’t have done the same. 

Reference:

Gómez-Consarnau, L., Akram, N., Lindell, K., Pedersen, A., Neutze, R., Milton, D. L., ... & Pinhassi, J. (2010). Proteorhodopsin phototrophy promotes survival of marine bacteria during starvation. PLoS biology, 8(4), e1000358.